Add support for self-contained data types

[divanchykhin]

A type is self-contained, if, after copying an object of that type, all references/pointers in its copy point to objects inside that copy.

Examples:

• numericals;
• owning pointer to an object of self-contained type;
• strings;
• data structures consisting of the above

Sufficient condition: no "magic 5" (c(d)tors) and all members are also self-contained.

Implementation: potentially can go along lines of adding an attribute or constexpr.

Also: respective rule should be added.

[marcosbracco]

See #34
This was replace by [[deep_const]], mostly the same but owning_ptr<const T> are only allowed.